The United States Department of Defense requires the use of MIL-STD-1553B as the standard for communications between electronic modules on all military airplanes and other vehicles. MIL-STD-1553B defines an asynchronous serial, command/response digital data bus on which messages are time division multiplexed among users. The 1553 STD specifies all of the electrical characteristics of receivers, as well as the complete message transmission protocol. The 1553 protocol specifies that all continuous data transfers start with a positive excursion of the wave form from a null bus condition (no significant signal present). The 1553 protocol further requires a "no response time out" of 14 microseconds and a 12-microsecond maximum allowed for remote terminal (RT) response time. This system will therefore not tolerate an additional signal propagation delay of more than 2 microseconds. The transmission medium is a twisted wire cable pair. The standard uses bi-phase Manchester II encoded signals for which timing of the zero-crossing signals is critical.
There is a need in the art to permit several 1553 remote terminals to be operated on a secondary bus that forms a stub off of a main bus. There is a further need to permit the use of the 1553 standard in a system where the length of a single main bus, or the loading on it, might cause the bus performance to become marginal or undependable. Finally, there is a need to permit the extension of a 1553 application to farther locations without modifying an existing physical bus structure.
In order to satisfy such needs in the art, signal repeaters have been devised to extend the application of the 1553 standard. For the purposes of this application, a repeater is a device that receives an encoded input data signal from a first bus, decodes and stores the data temporarily, then encodes and retransmits the data out on a second bus. Such a repeater is produced by Cal Corporation of 1050 Morrison Drive, Ottawa, Ontario, Canada. However, these devices have specific disadvantages in terms of high data latency and messaging complexity, requiring additional software/firmware and more complex electronics than the present invention. Furthermore, such repeaters are unable to work within the 1553 specifications due to the long propagation delay (more than 2 microseconds) through the repeater. There is therefore a demand for a device that will meet the needs described above, while staying within the 1553 specifications.